The invention relates to a luminescent material, a luminescent screen and a low pressure mercury lamp comprising such a luminescent screen. The invention also relates to a method for preparing the luminescent material.
In U.S. Pat. No. 4,703,224 a low pressure mercury discharge lamp for tanning purposes is disclosed comprising a luminescent screen containing, in addition to a UV-B phosphor, Sr.sub.2 P.sub.2 O.sub.7 :Eu.sup.2+ and Ba.sub.2 P.sub.2 O.sub.7 :Eu.sup.2+ as UV-A phosphors. This composition of the luminescent screen allowed the low pressure mercury discharge lamp (further also called lamp) to simulate the spectrum of the sun in the UV-region. A disadvantage of the used luminescent materials, however, is the fact that they both absorb radiation with a wavelength between 280 nm and 350 nm relatively strongly. As a result the short-wavelength radiation generated by the luminescent screen is to a relatively large extent reabsorbed and converted into long-wavelength radiation. The amount of short-wavelength radiation reabsorbed by the luminescent screen is a very strong function of its thickness. For this reason thickness variations in the luminescent screen along the lamp vessel of the lamp cause relatively large differences between the spectra of the light emitted from different places on the surface of the lamp vessel.